Studies on the taxonomy of Strongyloides (Nematoda; Strongyloididae)

Abstract

The taxonomy of Strongyloides has been critically assessed, firstly from the viewpoint of nomenclature, and subsequently from the aspect of morphology with emphasis placed on the practical problems of differentiating species. On the higher taxon level, the classification of the genus was discussed and placement in the Rhabdiasoidea favoured, although it was acknowledged that this was a compromise based on lack of knowledge of the Rhabdiasidae. The argument was presented that the valid name of the genus is Strongiloides, not Strongyloides, but that adoption of the former name would lead to instability without benefit. One hundred and three names used for species were located in the literature. Fifty three were considered valid, 18 invalid and 32 unavailable. Of this latter group, 22 were lapsi calamorum, 5 were nomina nuda, two had unacceptable spelling, and three lacked a differential diagnosis. The only species names which were considered invalid and are in common use were s.ransomi and S.planiceps, junior synonyms of s.suis and s.cati respectively. The genus was defined by description of the eighteen life cycle stages. This was based on Little (1966a,b) and some additions and corrections made to his basically sound definition of Strongyloides. The proposal was made that the parasitic female lacks cephalic papillae. Some changes in the limits of dimensions of the parasitic female were made, and it was emphasized that the maximum width relative to length, the distance of the vulva from the mouth relative to length, and the intramucosal location of the parasite are significant generic characters. The existence of perivulval nerve endings in the parasitic female was noted. The definition of the free living adults was essentially unchanged from that of Little, with the exception that the midventral preanal papilla of the free living male differs from the six paired caudal papillae. The existence of a papilla on the midpoint of the anterior cloacal lip was confirmed. Artifactual changes in all adult stages were described. The most common were degeneration due to death of worms or their host and those caused by the immune response of the host. The significance of artifactual changes in the taxonomy of Strongyloides was addressed, with particular reference to unusual features described in the literature for various species of Strongyloides. The criteria used to differentiate species in the genus were critically assessed. Those of most use were the stomal shape in the en face view and the ovary. type of the parasitic female, the distribution of caudal papillae in the free living male and features of gubernaculum and spicules, the post vulval constriction and posterior rotation of the vulva in the free-living female, and the stage of the parasite found in freshly voided faeces. Minor criteria were the shape of the tail in the parasitic female, the higher taxon classification of the host, and the occurrence of autoinfective larvae. Practical problems arising in the identification of unknown specimens were discussed. A significant problem not solved by this thesis is that 4l of the 53 valid species have not been adequately described. Consequently, an unusual approach to identification of unknown specimens was developed. This involved the use of a comprehensive host-Strongyloides list to demarcate a series of selection groups comprised of different species. The unknown specimen is compared with the first selection group, and points of similarity noted. Comparison then proceeds through the selection groups whose base broadens progressively. In this way, poorly described species are not omitted from the differential diagnosis. An attempt was made to apply these principles to the Strongyloides sp infecting man in Papua Nuigini. Available information indicated it was most consistent with S.fuelleborni. The nett effect of this thesis is a nomenclatural spring cleaning of the species in the genus, a precise definition of the genus with a clearer demarcation of generic characters, clarification of the significance of artifacts on the morphology useful for taxonomy, delimitation of those characters of use in differentiating species, and proposal of a practical scheme for identifying unknown specimens.